Polyolefins such as propylene polymers are so excellent in rigidity, heat resistance and impact resistance that they are used as various molded articles and in a variety of fields.
Known methods for-further enhancing the properties, such as the impact resistance, of polyolefins include, for example, one comprising first homopolymerizing propylene and subsequently copolymerizing propylene and ethylene to form a block copolymer.
Japanese Patent Laid-open Publication No. 4(1992)-337308 discloses the production of a block copolymer comprising first polymerizing propylene optionally with ethylene in the presence of a specified metallocene compound and an organoaluminum compound to obtain a propylene homopolymer or a propylene copolymer containing less than 6% by weight of ethylene units, and subsequently copolymerizing ethylene and propylene in a weight ratio of 10:90 to 95:5 to obtain an ethylene/propylene copolymer, wherein the homopolymer or copolymer obtained in the first step amounts to 40 to 95% by weight and the copolymer obtained in the second step amounts to 60 to 5% by weight, both being based on the total weight of the finally obtained block polymer. This block copolymer is described as having an excellent balance of impact resistance and rigidity.
Japanese Patent Laid-open Publication No. 5(1993)-202152 discloses a process for producing a polypropylene molding material comprising 20 to 99% by weight of a crystalline polymer having at least 95% by weight of propylene units (1) and 1 to 80% by weight of an amorphous ethylene/propylene copolymer having 20 to 90% by weight of ethylene units (2) in the presence of a catalyst composed of a transition metal compound and an organoaluminum compound, wherein the polymerization for obtaining the amorphous ethylene/propylene copolymer is carried out in the presence of a specified bridge type metallocene compound and aluminoxane. The polypropylene molding material obtained by this process is described as being excellent in, especially, low-temperature impact strength.
Known methods for further enhancing the impact resistance of olefin polymers also include one comprising blending an elastomer with polypropylene. However, due to its poor optical properties such as transparency, the use of such polypropylene having an elastomer blended therein has been limited.
The blending of atactic polypropylene for improving the properties of polypropylene is described in, for example, Japanese Patent Laid-open Publication No. 6(1994)-263934.
In recent years, the requirements on the properties of polyolefins are becoming more severe, and thus the developments of polyolefin compositions having further improved properties and molded articles of such polyolefin compositions are demanded.
The present invention has been made in the above circumstances. Thus, it is an object of the present invention to provide a polyolefin composition which is excellent in mechanical strength, flexibility and transparency, and it is another object of the present invention to provide a molded article of the polyolefin composition.